What: Shares of personal finance company BankRate (RATE)  are plunging by more than 46% at 2:00 p.m. ET as the market reacts to its fourth-quarter earnings, which missed analyst expectations by a mile.

So what: BankRate operates a number of personal finance websites that help consumers compare interest rates on anything from certificates of deposits to mortgages and credit cards. The company blamed changes to Google's search engine algorithms for declining advertising revenue.

Notably, fourth-quarter revenue came in at $93.4 million, significantly below Wall Street's consensus estimate of $134 million.

Kenneth Esterow, the company's president and CEO, said, "First, during the quarter, Google tested placement of a version of its Compare service that took up more of the display on credit card-related search results. Second, deposit advertisers significantly reduced their ad budgets far beyond the anticipated seasonal slowdown."

He noted that Google will discontinue its credit card comparison product in March, which should be good for BankRate's card comparison website. Clarity on Fed policy should help it drive advertising revenue on its CD pages, according to conference call commentary. Banks reduced their ad spending after the Fed raised interest rates for the first time last year. 

Now what: Analysts have slashed their price targets and ratings for BankRate on the back of its fourth-quarter earnings. RBC Capital downgraded the stock to "sector perform" and slashed its price target to $9 from $16. Topeka Capital Markets cut it to "hold" and also lowered its target to $9 from $16.

On the conference call, BankRate executives noted that Google changes could hurt its results in the short term, pointing out that the "shifting search landscape" may lead to an increase of 2 to 3 percentage points in advertising investments as a percentage of revenue, on top of the 5 percentage points of revenue it currently spends.